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Lecture 6

BIOLOGY 1M03 Lecture 6: 6- HWE and Types of Natural Selection

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Ben Evans

Bio 1M03 January 18, 2016 HWE and Types of Natural Selection Blood Types in Humans - Most human population have two co-dominant alleles for the MN blood group - Gene that governs the expression of blood (MN locus) - How to test if this locus is in HWE o Record the genotype frequencies by observation o Calculate allele frequencies from the observed genotype frequencies o Use the allele frequencies to calculate the expected genotype frequencies assuming HWE o Compare the observed and the expected genotype frequencies, if they don’t then we cannot refuse null hypothesis but If they’re drastically different there is bias and the null hypothesis can be rejected o When we cannot reject null hypothesis there isn’t much to say about the system o When it can be rejected, then there is something influencing it - Since the genotypes at the MN locus are similar to the HWE we can conclude that the factors (selection, drift, migration, mutation, biased mating) do not substantially affect the genotype frequencies. - This does not mean that MN blood types have always been in HWE - One generation of random mating can generate HWE genotypes for an autosomal locus (a gene not located on the sex chromosomes) - Only 2 ways to depart from HWE, either too many heterozygotes or too few or too many or too few homozygotes - We do not prove null hypothesis we either reject or approve the null hypothesis - Another point worth noticing is that allele frequencies of different human populations are different. This is suggestive of population structure between localities even though mating within localities appear within a locality. - The alleles frequencies between races and cultures all have radically different genotype frequencies - Variation across the world exists and causes the differences between the MN blood compositions HLA locus and the Immune Function - The human leukocyte antigen (HLA) system is a name for the major histocompatibility locus in humans - HLA genes are a set of genes on chromosome 6 that encode proteins that bind to antigens and present them on the outside of cells so the cells can be recognized by the immune system - Find pathogens, bind to them and kill those pathogens. - Diversity in the amino acid sequences contributes to fitness - The more variation at the protein level the better your immune system - Heterozygosity is advantageous, could mate choice increase Herterozygosity at HLA o Steps involved in the experiment 1. Ask some male student to not change their t-shirt for a few days 2. Ask female students to smell their shirts and rate them 3. Compare preferences to HLA genotypes of individuals that wore the T- shirt and the ones that evaluated them. o Advantageous for people to mate with hetero or opposite o Those not on the pill of those who did not have a shared allele more pleasant o Those who were taking oral contraceptives found the opposite effect, they found that they prefer the odor of the shared allele o Taking contraceptives derails the natural selection ethos o Null hypothesis: women should have no preference for either t-shirt category, regardless of shared allele o Null hypothesis: there is no different between the mate choice regardless of contraceptives - Could there be an advantage to Herterozygosity to HLA o Looked at couples who were attempting to get pregnant and looked at how long it took to get pregnant o Compared the time difference between if they shard or did not share the HLA alleles o Showed that individuals that shared a genotype were able to conceive. o Those who did not share the allele took longer to get pregnant o Heterozygous advantage o Null hypothesis: there should be no difference in the period of time it takes to get pregnant o Herterozygosity is advantageous - Hardy-Weinberg Equilibrium o To test the hypothesis that heterozygous for the HLA-A and HLA-B genes might be more fit than homozygous, researchers used genotype of 125 Havasupai tribe members to estimate population allele frequencies o If people tend to reproduce with partners with different HLA genotypes, what is the expectation with respect to HWE ▪ The expected values do not too greatly depart from the equilibrium but did not match ▪ The expected genotype frequencies did not match the observed frequencies ▪ More specifically there were more heterozygotes observed than expected by chance ▪ Therefore at least one of the Hardy- Weinberg assumptions must be violated for these alleles in this population o Alleles tende
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